The present invention relates to a wavelength control device for controlling a center wavelength by band narrowing of a laser light.
A band narrowing art for narrowing a band of a laser light oscillated from an excimer laser device and the like and controlling its center wavelength to be a desired value is conventionally known, and it is shown, for example, in Japanese Patent Application Laid-open No. 5-283785. FIG. 10 shows a configuration of a laser device disclosed in Japanese Patent Application Laid-open No. 5-283785, and the prior art will be explained hereinafter based on FIG. 10.
In FIG. 10, an excimer laser device 11 includes a laser chamber 12 for containing a laser gas being a laser medium, with windows 17 and 19 for transmitting a laser light 21 being attached at both end portions thereof. Inside the laser chamber 12, high voltage is applied between discharge electrodes not shown to excite the laser gas by electric discharge and to thereby generate the laser light 21.
The generated laser light 21 enters a band narrowing unit 30, expanded by a prism 32, reflected by a wavelength selection mirror 34, and is incident on a grating 33 being a band narrowing optical component. At the grating 33, only the laser light 21 having a wavelength close to a predetermined center wavelength xcexc is reflected by diffraction. This is called band narrowing.
In this situation, the wavelength selection mirror 34 is mounted on a movable holder 36 the angle of which with respect to the laser light 21 is changeable by a pulse motor 40. Changing the orientation of the wavelength selection mirror 34 changes an incident angle of the laser light 21 with respect to the grating 33, which changes the center wavelength xcexc of the laser light 21 diffracted by the grating 33. Specifically, by changing the orientation of the wavelength selection mirror 34, the center wavelength xcexc of the laser light 21 oscillated can be controlled to be a desired target wavelength xcex0.
Further, the excimer laser device 11 takes out part of the laser light 21 by means of a beam splitter 22, and monitors the center wavelength xcexc of the laser light 21 by means of a wavelength monitor 37. A laser controller 13 outputs a command signal to the pulse motor 40 based on the monitored center wavelength xcexc to rotate the wavelength selection mirror 34, thereby controlling the center wavelength xcexc of the laser light 21 to be the desired target wavelength xcex0.
The laser light 21 the band of which is narrowed inside the band narrowing unit 30 is amplified inside the laser chamber 12 while reciprocating several times between the band narrowing unit 30 and the front mirror 16 for partly reflecting the laser light 21. Subsequently, it is radiated toward the front (leftward in FIG. 10) as the laser light 21 having the center wavelength xcexc.
However, the aforementioned prior art has the following disadvantages.
Specifically, when the excimer laser device 11 is used as a light source for exposure of a processing machine such as a stepper, it is sometimes necessary to suspend the discharge and stop laser oscillation for a fixed period of time for replacement of a wafer and reticle. In such a case, a property of an optical component itself such as the prism 32 changes, or fixture components (not shown) for fixing the optical components is extended or contracted due to reduction in temperature around the optical components, thus causing the center wavelength xcexc to deviate. Further, after the discharge is stopped for a while, a drift of the center wavelength xcexc occurs due to a change in a state of the discharge electrodes and the laser gas. This drift is called chirping. Furthermore, the processing machine (not shown) sometimes sends a command to change the target wavelength xcex0 in response to a change in the temperature, atmospheric pressure and the like of the surrounding atmosphere.
When the center wavelength xcexc has to be changed as described above, the laser controller 13 outputs a command signal to the pulse motor 40 and thereby changes the angle of the wavelength selection mirror 34 to change the center wavelength xcexc. However, response time of the pulse motor 40 is generally long, and it takes about a few and a few tens of m sec to reach a desired stroke length of the pulse motor 40 after receiving a command signal. Thus, by the time the center wavelength xcexc reaches the target wavelength xcex0 after the laser controller 13 issues a command, a few pulses or a few tens of pulses of the laser light 21 are radiated if the laser light 21 is oscillated at a high pulse frequency of, for example, a few kHz.
As the result, the laser light 21 with the center wavelength xcexc unsuitable for exposure is incident on the processing machine, and defective exposure occurs. In order to avoid the above, the laser light 21 has to be blocked by, for example, a shutter or the like, but due to the time required to open and close the shutter, waiting time until the processing starts becomes longer, thus causing the disadvantage of reducing the availability of the processing machine.
The present invention is made to eliminate the disadvantages of the above art, and its object is to provide a wavelength control device for a laser device capable of controlling a center wavelength of a laser light to be a target wavelength in a short time.
In order to attain the above-described object, the wavelength control device for a laser device according to the present invention is a wavelength control device for a laser device including a movable holder for making an optical component movable with respect to a laser optical axis, and a laser controller for moving the optical component with respect to the laser optical axis and changing an incident angle of laser light on a band narrowing optical component to thereby control a center wavelength of the laser light to be a target wavelength, and is characterized in that
the movable holder comprises a first drive mechanism moving by a very short distance and a second drive mechanism moving by longer distance than the first drive mechanism.
According to the above configuration, when the center wavelength is finely controlled, the movable holder is driven by the first drive mechanism capable of moving by a very small distance, and when the center wavelength is moved to a large extent, it is driven by the second drive mechanism. Consequently, a wavelength control with high precision over a wide range is made possible.
Further, in the wavelength control device for the laser device,
the first drive mechanism may comprise a piezoelectric element unit, and
the second drive mechanism may comprise a pulse motor unit.
According to the above configuration, the piezoelectric element unit quick in response and the pulse motor unit with a long stroke are included. When the center wavelength has to be controlled in a short time, it is controlled with the piezoelectric element unit, and when it has to be controlled to a large extent, it is controlled with the pulse motor unit, thus making it possible to control the wavelength quickly over a wide range.
Further, in the wavelength control device for the laser device,
on moving the optical component, the laser controller may move the optical component by means of the piezoelectric element unit, and
thereafter, may move the optical component by means of the pulse motor unit.
According to the above configuration, since the optical component is initially moved by the piezoelectric element unit quick in response, the center wavelength quickly reaches the target wavelength. Thereafter, the control is performed by means of the pulse motor unit with a long stroke, and therefore the center wavelength can be changed to a large extent when the target wavelength is changed, thus facilitating response.
Further, in the wavelength control device for the laser device,
the laser controller
may move the optical component by means of the piezoelectric element unit to set the center wavelength at the predetermined target wavelength, and
may compensate a change of a position of the optical component caused by returning the piezoelectric element unit to a neutral position by means of the pulse motor unit, in a state in which the center wavelength being set at the target wavelength, at the same time when returning the piezoelectric element unit to the neutral position.
The piezoelectric element unit is sometimes deteriorated if a fixed high voltage keeps applied thereto for a long period of time. Accordingly, by returning the piezoelectric element to the neutral position, applied voltage does not become so high, thus preventing deterioration. When the piezoelectric element is returned to the neutral position, the positional change of the optical component is compensated by the pulse motor unit, thereby making it possible to always set the center wavelength at the target wavelength.
Further, in the wavelength control device for the laser device,
a wavelength monitor for monitoring the center wavelength of the laser light is further included, and
on resuming laser oscillation after stopping the laser oscillation for more than a predetermined period of time, the laser controller
may drive the pulse motor unit and thereby changes a position of the optical component with respect to the laser optical axis previously while the oscillation is stopped, based on the target wavelength after resuming the oscillation, and
may drive the movable holder by means of the piezoelectric element unit and thereby changes the position of the optical component with respect to the laser optical axis again, immediately after resuming the oscillation, based on the center wavelength of the laser light monitored by the wavelength monitor.
When the laser oscillation is stopped for more than a predetermined period of time, the center wavelength is sometimes deviated to a large extent from the target wavelength. Accordingly, in such a case, the position of the optical component is previously changed by the pulse motor unit, thereby almost setting the center wavelength at the target wavelength. In addition, the wavelength control is resumed by the piezoelectric element unit fast in response, thus reducing the time that elapses before the center wavelength is set at the target wavelength.